This section is intended to provide a background to the various embodiments of the invention that are described in this disclosure. Therefore, unless otherwise indicated herein, what is described in this section should not be interpreted to be prior art by its mere inclusion in this section.
Radio communication networks are widely deployed to provide various communication services such as telephony, video, data, messaging, broadcasts, and so on. Such radio communication networks support communications for multiple user equipments (UEs) by sharing the available network resources. One example of such a network is the Universal Mobile Telecommunications System (UMTS), a third generation (3G) technology standardized by the 3rd Generation Partnership Project (3GPP). UMTS includes a definition for a Radio Access Network (RAN), referred to as UMTS Terrestrial Radio Access Network (UTRAN). The UMTS, which is the successor to Global System for Mobile Communications (GSM) technologies, supports various air interface standards, such as Wideband-Code Division Multiple Access (W-CDMA), Time Division-Code Division Multiple Access (TD-CDMA), and Time Division-Synchronous Code Division Multiple Access (TD-SCDMA). The UMTS also supports enhanced 3G data communications protocols, such as High Speed Packet Access (HSPA), which provides higher data transfer speeds and capacity to associated UMTS networks. As the demand for mobile broadband access continues to increase, research and development continue to advance the UMTS technologies not only to meet the growing demand for mobile broadband access, but to advance and enhance the user experience with mobile communications. For example, third-generation UMTS based on W-CDMA has been deployed in many places of the world. To ensure that this system remains competitive in the future, 3GPP began a project to define the long-term evolution of UMTS cellular technology. The specifications related to this effort are formally known as Evolved UMTS Terrestrial Radio Access (E-UTRA) and Evolved UMTS Terrestrial Radio Access Network (E-UTRAN), but are more commonly referred to by the name Long Term Evolution (LTE). More detailed descriptions of radio communication networks and systems can be found in literature, such as in Technical Specifications published by, e.g., the 3GPP.
Carrier Aggregation (CA)
Carrier aggregation (CA) can be utilized in radio communication networks in order to increase the bandwidth, and thereby increase the bitrates. Each aggregated carrier is generally referred to as a component carrier (CC). The component carrier can have a bandwidth of 1.4, 3, 5, 10, 15 or 20 MHz and a maximum of five component carriers can be aggregated, hence the maximum aggregated bandwidth is generally 100 MHz. In Frequency Division Duplex (FDD), the number of aggregated carriers can be different in downlink (DL) and uplink (UL). However, the number of UL component carriers is generally equal to or lower than the number of DL component carriers. The individual component carriers can also be of different bandwidths. When Time Division Duplex (TDD) is used, the number of CCs and the bandwidth of each CC are generally the same for DL and UL.
One way to arrange the carrier aggregation is to use contiguous CCs within the same operating frequency band (as defined for LTE), so called intra-band contiguous. This might however not always be possible, due to operator frequency allocation scenarios, etc. For non-contiguous allocation, it could either be intra-band (i.e. the component carriers belong to the same operating frequency band) but have a gap, or gaps, in between. Alternatively, it could be inter-band, in which case the component carriers belong to different operating frequency bands.
When carrier aggregation is used there is generally a number of serving radio cells, one for each component carrier. The coverage of the serving radio cells may differ, both due to component carrier frequencies but also from power planning (which is e.g. useful for heterogeneous network planning). Generally, the RRC (Radio Resource Control) connection is only handled by one cell, the Primary serving cell, served by the Primary component carrier (DL and UL PCC (Primary CC)). It is also on the DL PCC that the UE receives Non-Access Stratum (NAS) information, such as security parameters. In idle mode the UE listens to system information on the DL PCC. On the UL PCC, the PUCCH (Physical Uplink Control Channel) is sent. The other component carriers are all referred to as Secondary component carriers (DL and UL SCC), serving the Secondary serving radio cells. Generally, the SCCs (secondary carrier) are added and removed as required, while the PCC (primary carrier) is only changed at handovers.
Different component carriers can be planned to provide different coverage, i.e. different radio cell size. In the case of inter-band carrier aggregation the component carriers may experience different pathloss, which may increase with increasing frequency. Furthermore, the introduction of carrier aggregation may influence Medium-Access Control (MAC) and the physical layer protocol, but also some new RRC messages are introduced. In order to keep compatibility with earlier 3GPP releases (e.g. Release 8 (Rel8)/Release 9 (Rel-9)), the protocol changes will most likely be kept to a minimum. Basically each component carrier may be treated as a Rel-8 carrier. This way, backwards compability is catered for. Hence, to a Rel-8 UE or a Rel-9 UE (i.e. UEs supporting 3GPP Rel-8 or Rel-9, respectively) each CC will appear as a Rel-8 carrier, while a CA-capable UE can exploit the total aggregated bandwidth, enabling higher data rates.
More detailed descriptions of CA can be found in literature, such as in Technical Reports and Specifications published by the 3GPP. For further reading about CA see for example 3GPP TR 36.808, 3GPP TR 36.814, 3GPP TR 36.815, 3GPP TR 36.823, 3GPP TR 36.912, 3GPP TR 36.913, 3GPP TS 36.211, 3GPP TS 36.212, 3GPP TS 36.213, and 3GPP TS 36.300.
Classical Versus Combined Radio Cell Deployments
In the following, the term point is used to mean a point having transmission and/or reception capabilities. As used herein, this term may interchangeably be referred to as “transmission point”, “reception point”, “transmission/reception point” or “node”. To this end, it should also be appreciated that the term point may include devices such as radio network nodes (e.g. evolved NodeB (eNB), a Radio Network Controller (RNC), etc)) and radio units (e.g. Remote Radio Units (RRUs)). As is known among persons skilled in the art, radio network nodes generally differ from RRUs in that the radio network nodes have comparatively more controlling functionality. For example, radio network nodes typically include scheduler functionality, etc., whereas RRUs typically don't. RRUs are typically consuming comparatively less computational power than radio network nodes. Sometimes, radio network nodes may therefore be referred to as high power points or high power nodes (HPN) whereas RRUs may be referred to as low power points or low power nodes (LPN). In some cell deployments, LPNs are referred to as pico points and HPNs are referred to as macro points. Thus, macro points are points having comparatively higher power than the pico points.
Heterogeneous deployments generally refer to deployments having a mixture of radio cells having different DL transmission power, operating on (at least partially) the same set of frequencies and with overlapping geographic coverage, as is schematically illustrated in FIG. 1. A typical example is a so-called pico cell placed within the coverage area of a comparatively larger radio cell, i.e. the macro cell. The classical way of deploying a radio communication network is thus to let different transmission/reception points form separate cells. That is, the signals transmitted from or received at a transmission/reception point is associated with a cell-id (e.g. a Physical Cell Identity (PCI)) that is different from the cell-id employed for other nearby points. Conventionally, each point transmits its own unique signals for broadcast (PBCH (Physical Broadcast Channel)) and sync channels (PSS (primary synchronization signal), SSS (secondary synchronization signal)). Note that similar principles also apply to classical macro-cellular deployments where all points have similar output power and perhaps are placed in a more regular fashion compared with the case of a heterogeneous deployment.
A recent alternative to the classical cell deployment is to instead let all the UEs within the geographical area outlined by the coverage of the high power point be served with signals associated with the same cell-id (e.g. the same Physical Cell Identity (PCI)). In other words, from a UE perspective, the received signals appear coming from a single radio cell. This is illustrated in FIG. 2 and is generally referred to as combined cell deployment, shared cell deployment or soft cell deployment. Note that only one macro point is shown, other macro points would typically use different cell-ids (corresponding to different radio cells) unless they are co-located at the same site (corresponding to other radio cell sectors of the macro site). In the latter case of several co-located macro points, the same cell-id may be shared across the co-located macro points and those pico points that correspond to the union of the coverage areas of the macro points. In a combined radio cell, the comparatively larger radio cell 10c may be referred to as the radio cell whereas the smaller radio cells 20-1c, 20-2c, 20-3c may be referred to as radio cell sectors or sectors. Sync channels, BCH (Broadcast Channels) and control channels may all be transmitted from the high power point while data can be transmitted to a UE also from low power points by using shared data transmissions (e.g. a Physical Downlink Shared Channel (PDSCH)) relying on UE specific resources.
The single cell-id approach, or combined cell deployment, can be geared towards situations in which there is fast backhaul communication between the points associated to the same cell. A typical case would be a base station serving one or more sectors on a macro level as well as having fast fiber connections to remote radio units (RRUs) playing the role of the other points sharing the same cell-id. Those RRUs could represent low power points with one or more antennas each. Another example is when all the points have a similar power class with no single point having more significance than the others. The base station would then handle the signals from all RRUs in a similar manner.
An advantage of the combined cell deployment compared with the classical deployment is that the typically involved handover procedure between cells may only need to be invoked on a macro basis. Moreover, there is generally also greater flexibility in coordination and scheduling among the points which means the network can avoid relying on the potentially inflexible concept of semi-statically configured “low interference” subframes as in Rel-10. A combined cell approach also allow decoupling of the downlink (DL) with the uplink (UL) so that for example path loss based reception point selection can be performed in UL while not creating a severe interference problem for the DL, where the UE may be served by a transmission point different from the point used in the UL reception.
It should be appreciated that a combined cell deployment may bring further advantages when used in combination with Spatial Division Multiplexing (SDM). For example, assume a radio cell having three radio sectors employing 20 MHz (megahertz) bandwidth with 100 PRBs (Physical Resource Blocks), all the radio sectors share the 100 PRBs if SDM is not supported. However, if SDM is supported all three radio sector may be isolated by means of SDM and, as a consequence, the three radio sectors can utilize 100 PRBs each, thus increasing the overall throughput of the combined radio cell.